Knife gate valves were originally designed to be used in paper-pulp flow systems, with the gates thereof having sharp blades at the lower ends to cut through the pulp. However, since their first introduction, the use of knife gate valves has extended into many other fields. In this respect, knife gate valves are commonly used to control the flow of coal slurry, sewage, water, vacuums, chemicals, and the like.
A problem with prior-art knife gate valves is that it is expensive and cumbersome to make them "bubble-tight" (so that there is virtually no water seepage therethrough). For this reason, where there has been no necessity that such valves be "bubble-tight", they have not been. Applications which do not require "bubble-tightness" include coal slurry and sewage applications. Similarly, even where "bubble-tightness" has been required, gate valves have normally been made so that they provide such sealing for flows in only one direction. In such systems, back surges provide seepage through most prior-art knife gate valves, even those which are "bubble tight".
In order to provide "general-application" knife-gate valves, it has been common practice for some manufacturers to manufacture knife gate valves which do not have "bubble-tight" seals, but on which "bubble-tight" seals can be installed. However, in such a case, the seal is often placed in the liquid flow passage of the valve, and therefore cuts down on the passage cross-sectional area.
Further, a problem that is associated with lack of "bubble-tightness" on the upstream side of a knife-gate valve is that sewage, chemicals, or the like that is being controlled by the valve can often flow up along the blade of the valve, and into the valve mechanisms, thereby caking, corroding or otherwise interferring with these mechanisms and making the valve difficult to open and close.
It is therefore an object of this invention to provide a gate valve which provides a "bubble-tight" seal for knife gate valves for flows in both directions.
Similarly, it is an object of this invention to provide a gate valve in which liquid that is being controlled by the valve is essentially prevented from passing along the valve's blade to mechanisms for opening and closing the valve.
Along with the preceding two objects, it is also an object of this invention to provide a gate valve which provides a "bubble-tight" seal without reducing the cross-sectional area of a liquid flow passage.
It has been common practice in the prior-art to provide a gate-blade actuator for knife gate valves which includes an external wheel threaded onto a screw attached to the gate blade. The wheel is held against axial movement. When the wheel is rotated the screw is caused to move axially in the wheel and thereby move the gate blade upwardly and downwardly. Such a mechanism in which the screw moves upwardly and downwardly is termed in this application a "rising" mechanism. In such a mechanism packing around the gate blade prevents flow of liquid up to the screw mechanism. However, such an arrangement is sometimes inconvenient because the moving screw is cumbersome. Thus, it is an object of this invention to provide a knife gate valve having a gate blade drive mechanism of the "non-rising" type in which a threaded screw does not move up and down when the valve is being opened and closed.
In many applications of knife gate valves, the gate valves only have a life that is measured in months. For example, in coal slurry applications many valves must be totally removed from pipe systems and replaced after only four months of operation. Since these valves cost in the hundreds of dollars and are quite heavy, their frequent replacement is expensive and time consuming. Some knife gate valves have been produced which have removable liners held in frames in which gate blades move up and down. However, in these systems the gate-valve frames must be removed from, or at least significantly loosened from pipe systems to which they are connected to replace the liners. This is also expensive.
It is therefore an object of this invention to provide a gate valve having a liner which can be periodically replaced, without removing a valve frame from the pipe system to which the valve frame is connected.
Similarly, it is an object of the invention to provide a gate valve having a life span which is considerably longer than prior-art gate valves.
It is yet another object of this invention to provide a gate valve which is relatively inexpensive to manufacture, but which is universal in application, since it can be beneficially used in both cases where "bubble-tight" seals are and are not required.
Further, it is an object of this invention to provide a gate valve in which the direction in which the valve is installed is not important, since the valve provides a "bubble-tight" seal in both directions.
It is a further object of this invention to provide a gate valve which operates relatively easily over an extended period of use.
In addition, it is an object of this invention to provide a knife gate valve whose liner, and valve seat are fully protected by metallic shields against impact from hard objects (such as coal) passing through the valves.
It is still another object of this invention to provide a gate valve whose liner can be constructed in the field by a mechanic, when necessary.